Been there twice, including this past summer. We dr never been to the north, but we've seen most of Andalusia and spent a week in Barcelona. Barcelona's a nice city, but we didn't love it like we did Seville.
Andalusia is REALLY hot in August (we saw 46C) but it's very dry, so the heat is manageable. A lot of shops are closed for vacation and museum/etc hours are cut back, but nothing that will really impact your stay.
Definitely check out the Paradores. We stayed in the one in Carmona for 2 nights and it was amazing. And that one apparently doesn't even make the top 10. The restaurant there was only ok, though, for dinner. Much better places to eat in town. Though the breakfasts are awesome, included in the price, and delivers to your room free of charge (if you want). Having breakfast in our bathrobes in front of the open balcony looking over the Spanish countryside, with a slight hangover from wine and, well, what couples do on their anniversary
is one of our favourite memories of all our trips.